1. Field of the Invention
The present invention relates to methods for producing hydrogen and for removing and sequestering carbon dioxide (CO2) from coal-fired furnaces and boilers.
2. Description of Related Art
Carbon dioxide (CO2), one of the so-called greenhouse gases, is produced during the combustion of fossil fuels, especially coal, in furnaces and power plants. Recent scientific studies have shown that emissions of CO2 and other greenhouse gases, which include methane (CH4), sulfur dioxide (SO2), and nitrogen oxides (NOx), can have a significant effect on climate change. The prospect of climate change caused, at least in part, by emission of CO2 and other greenhouse gases has led to international concern and to international treaties, such as the Kyoto Protocol. At the time of writing, the Kyoto Protocol has been approved but has not gone into effect because of some international resistance.
The Kyoto Protocol specifies that the industrialized nations of the world are to reduce CO2 emissions to 7% below 1990 levels. In addition to that requirement, the Kyoto Protocol allows “emission trading,” under which countries with higher emission levels of greenhouse gases can buy “emission credits” from countries that are not emitting their allotted levels of greenhouse gases. Various “emission trading” schemes have also been in use within the United States for some time.
Because of the national and international concern, power producers have been attempting to reduce the levels of CO2 produced by power plants, particularly coal-fired power plants. Many newer power plants are combined cycle plants fired by natural gas, which produce significantly less CO2. However, because of the relative abundance of coal in the United States and the fact that there have been significant construction delays in building new gas fired power plants, methods to reduce the emission of coal-fired plants are needed.
Some methods have been proposed for sequestering the CO2 produced by coal fired and other power plants. One method for CO2 sequestration involves reacting the CO2 with large quantities of metal oxides, particularly calcium oxide and magnesium oxide, then burying the resulting carbonates. This method requires a large and continuous supply of minerals in order to sequester CO2. Another proposed method involves removing the CO2 from the combustion gas at the power plant, compressing it, shipping it by pipeline to a peridotite or serpentinite mine for conversion to a carbonate, and then burying it at the mine site. However, shipment of compressed CO2 may be a materials handling problem, because CO2, which is heavier than air, will stay close to the ground if it is accidentally released during transit, and can thus pose a danger to life. Additionally, availability of mines may be a problem. Other methods involve compression of CO2 and injection into the sea.
In general, these methods for sequestering CO2 do not take into account the overall efficiency of the power plant, which may be reduced by up to 30% if they are implemented, nor do they take into account certain economic considerations. In order for a method for reducing CO2 emissions to be desirable, it should be lower in cost, and it should not damage the interests of current power stakeholders, or it may be blocked from implementation.
Some work has been done on new power cycles for coal power plants. For example, a zero emissions coal fired power plant can incorporate the above-described mineral sequestration method for CO2 with a different power cycle. It consists of a coal gasification step that produces CH4, H2, and CO2, followed by a carbonation step that uses CH4 and H2 and uses CaO to capture the CO2. The hydrogen “fires” a fuel cell. The CO2 is sequestered either by the mineral process or by injection in other geologic sites, such as depleted oil wells or coalmines or saline aquifers or the ocean. The CaO is calcined from mined mineral. However, one of the key technical development issues with this proposed method is a fuel cell that is tolerant of sulfur compounds released by the coal gasification, and, as such, the fuel cell may be more difficult to develop than fuel cells fired with methane, i.e., natural gas (although natural gas also contains minute quantities of sulfur). Another issue is the possibility of inefficient heat transfer between many steps in this complex system, which may sharply reduce the feasibility of this cycle. Additionally, since no large central station power plants with methane fuel cells are in existence, the probability of developing even a full scale prototype plant for this much more complicated coal fired system in the near future is very small, and the possibility of building enough power plants of this type to meaningfully reduce CO2 emissions is even smaller.